The present invention relates to polyester synthase, a gene coding for the enzyme, a recombinant vector containing the gene, a transformant transformed with the vector, and a process for producing polyester synthase by use of the transformant.
Polyesters (e.g., poly-3-hydroxyalkanoic acid) biosynthesized by microorganisms are biodegradable plastics with thermoplasticity ranging widely from rigid matter to viscoelastic rubber.
Poly-3-hydroxybutanoic acid (P(3HB)) is a typical polyester consisting of C4 monomer units, but it is a rigid and brittle polymeric material, so its application is limited. Accordingly, various polyesters such as P(3HB-co-3HV) having (P(3HB)) copolymerized with a C5 monomer unit (3HV) by adding propionic acid etc. to the medium have been prepared and examined to alter the physical properties of the polyester. On the other hand, polyesters consisting of at least C6 monomer units are soft polymeric materials having plasticity.
Polyester-synthesizing microorganisms are roughly divided into 2 groups, that is, those synthesizing polyesters with C3-5 monomer units and those synthesizing polyesters with C6-14 monomer units. The former microorganisms possess a polyester synthase using C3-5 monomer units as the substrate, while the latter microorganisms possess a polyester synthase using C6-14 monomer units as the substrate. Therefore, polyesters with different properties are synthesized by the respective microorganisms.
However, the respective polyesters from such known microorganisms are different in substrate specificity, so with one kind of enzyme given, polyesters (copolymers) having various monomer unit compositions adapted to the object of use are difficult to synthesize.
The object of the present invention is to provide a polyester synthase with monomer units having a wide range of carbon atoms as the substrate, a gene coding for the enzyme, a recombinant vector containing the gene, a transformant transformed with the vector, and a process for producing the polyester synthase by use of the transformant.
As a result of their eager research, the present inventors succeeded in cloning a polyester synthase gene from a microorganism belonging to the genus Pseudomonas isolated from soil, to arrive at the completion of the present invention.
That is, the present invention is a polypeptide comprising the amino acid sequence of SEQ ID NO:1 or a sequence where in said amino acid sequence, one or more amino acids are deleted, replaced or added, said polypeptide having polyester synthase activity.
Further, the present invention is a polyester synthase gene comprising DNA coding for said polypeptide. The DNA coding for the protein with polyester synthase activity includes, e.g., that of SEQ ID NO:2.
Further, the present invention is a polyester synthase gene comprising the nucleotide sequence of SEQ ID NO:3.
Further, the present invention is a recombinant vector comprising the polyester synthase gene.
Further, the present invention is a transformant transformed with said recombinant vector.
Further, the present invention is a process for producing polyester synthase wherein said transformant is cultured in a medium and polyester synthase is recovered from the resulting culture.
Hereinafter, the present invention is described in detail.
(1) Cloning Of The Polyester Synthase Gene
The polyester synthase gene of the present invention is separated from a microorganism belonging to the genus Pseudomonas.
First, genomic DNA is isolated from a strain having the polyester synthase gene. Such a strain includes, e.g., Pseudomonas sp. Any known methods can be used for preparation of genomic DNA. For example, Pseudomonas sp. is cultured in a bouillon medium and then its genomic DNA is prepared by the hexadecyl trimethyl ammonium bromide method (Current Protocols in Molecular Biology, vol. 1, page 2.4.3., John Wiley and Sons Inc., 1994).
The DNA obtained in this manner is partially digested with a suitable restriction enzyme (e.g., Sau3AI, BamHI, BglII etc.). It is then ligated into a vector dephosphorylated by treatment with alkaline phosphatase after cleavage with a restriction enzyme (e.g., BamHI, BglII etc.) to prepare a library.
Phage or plasmid capable of autonomously replicating in host microorganisms is used as the vector. The phage vector includes, e.g., EMBL3, M13, gt11 etc., and the plasmid vector includes, e.g., pBR322, pUC18, and pBluescript II (Stratagene). Vectors capable of autonomously replicating in 2 or more host cells such as E. coli and Bacillus brevis, as well as various shuttle vectors, can also be used. Such vectors are also cleaved with said restriction enzymes so that their fragment can be obtained.
Conventional DNA ligase is used to ligate the resulting DNA fragment into the vector fragment. The DNA fragment and the vector fragment are annealed and then ligated to produce a recombinant vector.
To introduce the recombinant vector into a host microorganism, any known methods can be used. For example, if the host microorganism is E. coli, the calcium chloride method (Lederberg, E. M. et al., J. Bacteriol. 119, 1072 (1974)) and the electroporation method (Current Protocols in Molecular Biology, vol. 1, page 1.8.4 (1994)) can be used. If phage DNA is used, the in vitro packaging method (Current Protocols in Molecular Biology, vol. 1, page 5.7.1 (1994)) etc. can be adopted. In the present invention, an in vitro packaging kit (Gigapack II, produced by Stratagene etc.) may be used.
To obtain a DNA fragment containing the polyester synthase gene derived from Pseudomonas sp., a probe is then prepared. The amino acid sequences of some polyester synthases have already been known (Peoples, O. P. and Sinskey, A. J., J. Biol. Chem., 264, 15293 (1989); Huisman, G. W. et al., J. Biol. Chem., 266, 2191 (1991); Pieper, U. et al., FEMS Microbiol. Lett., 96, 73 (1992); Timm, A. and Steinbuchel, A., Eur. J. Biochem., 209, 15 (1992), etc.). Well-conserved regions are selected from these amino acid sequences, and nucleotide sequences coding for them are estimated to design oligonucleotides. Examples of such oligonucleotides include, but are not limited to, the sequence 5xe2x80x2-CC(G/C)CAGATCAACAAGTT(C/T)TA(C/G)GAC-3xe2x80x2 (SEQ ID NO:4) reported by Timm, A. and Steinbuchel, A., Eur. J. Biochem., 209, 15 (1992).
Then, this synthetic oligonucleotide is labeled with a suitable reagent and used for colony hybridization of the above genomic DNA library (Current Protocols in Molecular Biology, vol. 1, page 6.0.3 (1994)).
The E. coli is screened by colony hybridization, and a plasmid is recovered from it using the alkaline method (Current Protocols in Molecular Biology, vol. 1, page 1.6.1 (1994)), whereby a DNA fragment containing the polyester synthase gene is obtained. The nucleotide sequence of this DNA fragment can be determined in, e.g., an automatic nucleotide sequence analyzer such as 373A DNA sequencer (Applied Biosystems) using a known method such as the Sanger method (Molecular Cloning, vol. 2, page 13.3 (1989)).
After the nucleotide sequence was determined by the means described above, the gene of the present invention can be obtained by chemical synthesis or the PCR technique using genomic DNA as a template, or by hybridization using a DNA fragment having said nucleotide sequence as a probe.
(2) Preparation of Transformant
The transformant of the present invention is obtained by introducing the recombinant vector of the present invention into a host compatible with the expression vector used in constructing said recombinant vector.
The host is not particularly limited insofar as it can express the target gene. Examples are bacteria such as microorganisms belonging to the genus Alcaligenes, microorganisms belonging to the genus Bacillus, bacteria such as E. coli, yeasts such as the genera Saccharomyces, Candida etc., and animal cells such as COS cells, CHO cells etc.
If microorganisms belonging to the genus Alcaligenes or bacteria such as E. coli are used as the host, the recombinant DNA of the present invention is preferably constituted such that it contains a promoter, the DNA of the present invention, and a transcription termination sequence so as to be capable of autonomous replication in the host. The expression vector includes pLA2917 (ATCC 37355) containing replication origin RK2 and pJRD215 (ATCC 37533) containing replication origin RSF1010, which are replicated and maintained in a broad range of hosts.
The promoter may be any one if it can be expressed in the host. Examples are promoters derived from E. coli, phage etc., such as trp promoter, lac promoter, PL promoter, PR promoter and T7 promoter. The method of introducing the recombinant DNA into bacteria includes, e.g., a method using calcium ions (Current Protocols in Molecular Biology, vol. 1, page 1.8.1 (1994)) and the electroporation method (Current Protocols in Molecular Biology, vol. 1, page 1.8.4 (1994)).
If yeast is used as the host, expression vectors such as YEp13, YCp50 etc. are used. The promoter includes, e.g., gal 1 promoter, gal 10 promoter etc. To method of introducing the recombinant DNA into yeast includes, e.g., the electroporation method (Methods. Enzymol., 194, 182-187 (1990)), the spheroplast method (Proc. Natl. Acad. Sci. USA, 84, 1929-1933 (1978)), the lithium acetate method (J. Bacteriol., 153, 163-168 (1983)) etc.
If animal cells are used as the host, expression vectors such as pcDNAI, pcDNAI/Amp (produced by Invitrogene) etc. are used. The method of introducing the recombinant DNA into animal cells includes, e.g., the electroporation method, potassium phosphate method etc.
(3) Production of polyester Synthase
Production of the polyester synthase of the present invention is carried out by culturing the transformant of the present invention in a medium, forming and accumulating the polyester synthase of the present invention in the culture (the cultured microorganism or the culture supernatant) and recovering the polyester synthase from the culture.
A conventional method used for culturing the host is also used to culture the transformant of the present invention.
The medium for the transformant prepared from bacteria cush as E. coli etc. as the host includes complete medium or synthetic medium, e.g. LB medium, M9 medium etc. The transformant is aerobically cultured at a temperature ranging from 25 to 37 degrees C. for 12 to 48 hours so that the polyester synthase is accumulated in the microorganism and then recovered.
The carbon source is essential for the growth of the microorganism and includes, e.g., carbohydrates such as glucose, fructose, sucrose, maltose etc.
The nitrogen source includes, e.g., ammonia, ammonium salts such as ammonium chloride, ammonium sulfate, ammonium phosphate etc., peptone, meat extract, yeast extract, corn steep liquor etc. The inorganic matter includes, e.g., monopotassium phosphate, dipotassium phosphate, magnesium phosphate, magnesium sulfate, sodium chloride etc.
Culture is carried out usually under aerobic conditions with shaking at 25 to 37xc2x0 C. for more than 2 hours after expression is induced. During culture, antibiotics such as ampicillin, kanamycin, ampicillin, tetracycline etc. may be added to the culture.
To culture the microorganism transformed with the expression vector using an inducible promoter, its inducer can also be added to the medium. For example, isopropyl-D-thiogalactopyranoside (IPTG), indoleacrylic acid (IAA) etc. can be added to the medium.
To culture the transformant from animal cells as the host, use is made of a medium such as RPMI-1640 or DMEM which may be supplemented with fetal bovine serum. Culture is carried out usually in 5% CO2 at 30 to 37xc2x0 C. for 1 to 7 days. During culture, antibiotics such as kanamycin, penicillin etc. may be added to the medium.
Purification of the polyester synthase can be performed by recovering the resulting culture by centrifugation (after disruption in the case of cells) and subjecting it to affinity chromatography, cation or anion exchange chromatography or gel filtration or to a suitable combination thereof.
Whether the resulting purified substance is the desired enzyme is confirmed by conventional methods such as SDS polyacrylamide gel electrophoresis, Western blotting etc.